Thermal method of making oil gas



Oct. 18, 1955 v E. s. PETTYJOHN ETAL 2,721,122

THERMAL METHOD OF MAKING OIL GAS 3 Sheets-Sheet 1 Filed Sept. 24, 1952 1955 E. s. PETTYJOHN ET AL 2,721,122

THERMAL METHOD OF MAKING OIL GAS Filed Sept. 24, 1952 3 Sheets-Sheet 2 Oct. 18, 1955 E. s. PETTYJOHN ET AL 2,721,122

THERMAL METHOD OF MAKING OIL GAS Filed Sept, 24, 1952 4 3 Sheets-Sheet 3 United States Patent 1 2,721,122 THERMAL METHOD OF MAKING OIL GAS Elmore S. Pettyjohn, Evanstou, and Henry R. Linden,

Franklin Park, 111., assignors to The Institute of Gas Technology, Chicago, 111., a corporation of Illinois Application September 24, 1952, Serial No. 311,256

2 Claims. (Cl. 48211) This invention relates to a method of preparing an oil gas absolutely interchangeable with natural gas.

Reference is made to our copending application Serial No. 270,955, filed February 2, 1952, and which has issued as Patent No. 2,707,675, May 3, 1955, for a complete discussion of the interchangeability of various fuel gases with natural gas.

In the last 20 years, the use of natural gas by public utility companies has increased tremendously, to the extent where in 1949 some twenty or more per cent of the total energy consumed in the United States was furnished by natural gas. This natural gas has been made available at the various localities where it is burned by transmission through pipelines. As the use of natural gas has increased, the problem of meeting the seasonal peak load demands has become more and more difficult. This problem is brought about by the tremendous demand for gas on the peak load days, usually the coldest winter days, when several times the average daily sendout is required, and has been made particularly acute by a disproportionate rate of growth of gas sales for house and other space heating. It should be noted that long distance natural gas transmission lines can be built economically only to handle a steady high load. Up to the present time, there has been no really satisfactory solution to the peak load problem, in view of the fact that no gas absolutely interchangeable with natural gas has been available. Utility companies have, therefore, been forced to blend their sendout of natural gas with other gases only partially interchangeable with natural gas, with resultant unsatisfactory burning characteristics of the resultant mixture if excessive amounts of such other gases have been incorporated with the mixture. Further, even such only partially interchangeable gases have been produced only at excessive costs, either due to heavy capital investments for manufacturing apparatus used only to meet peak demands, or due to high costs of the gas manufacture apart from the required capital investment for apparatus, or due to a combination of these two factors.

It is, therefore, an important object of the present invention to provide a simple and relatively inexpensive method for making a fuel gas absolutely interchangeable with natural gas and capable of being carried out in apparatus of relatively low cost.

Other and further objects and features of the present invention will become apparent from the following description and appended claims as illustrated by the accompanying drawings showing, diagrammatically and by way of examples, apparatus for carrying out the methods of this invention. More particularly, the three figures of the drawings are vertical, longitudinal, sectional views (with parts shown in elevation) of three different types of apparatus for carrying out the methods of the present invention. The apparatus shown in the drawings is disclosed and claimed in the copending application of Elmore S. Pettyjohn entitled Oil Gas Plant Serial No. 270,956, filed February 11, 1952.

The starting material employed in the methods of the present invention is a petroleum fraction not more volatile than straight run naphtha. Examples of such petroleum fractions are straight run naphtha, light diesel, stove or No. 1 fuel oil, diesel or gas house gas oil, reduced crude, heavy gas oil or premium residuum, and fuel oil such as No. 2, No. 3, cold No. 5, No. 5, No. 6 and Bunker C fuel oil.

2,721,122 Patented Oct. 18, 1955 According to the present invention, the above disclosed starting materials are cracked thermally under a superatmospheric pressure ranging upwardly from two atmospheres to about four atmospheres absolute. The temperature is maintained at from 1300 to 17 50 or higher. The residence time may range from 15 to 3 or 2 seconds. Steam or nitrogen or combustion gas or oil gas or natural gas or like gases may be incorporated with material being cracked by way of a diluent.

For normal operating conditions within the indicated limits, the cracking step will yield a gas with a true or diluent free oil gas fraction containing from 12 to 15% ethylene, from 1 to 5% higher unsaturated hydrocarbons, from 10 to 20% hydrogen, from 55 to or more parafiins having an average carbon number ranging from less than 1.1 up to 1.2 and having a heating value of from 900 to 1300 B. t. u. per cubic foot as well as a specific gravity (air=l.000) ranging from 0.50 to 0.78. The liquid products are tars and light oils of a composition similar to the light oil obtained when, using the same starting hydrocarbon, oil gas is produced by conventional methods at atmospheric pressure and under conditions adjusted to form oil gas having the same diluent-free heating value.

The heating value and specific gravity of the oil gases produced according to the present invention may be adjusted by the addition of combustion gases to give such combustion chaarcteristics as may be desired.

In the case of straight run naphtha (or starting material not less volatile than straight run naphtha), the cracking can be carried out with a carbon deposition not above 2 /2 weight per cent of the material being cracked by maintaining a pressure of from two to three atmospheres absolute, a temperature of from 1300 to 1550 F. and a residence time of from 5 to 2 seconds. Under these conditions, straight run naphtha can be cracked thermally by simply continuously passing the naphtha (in vapor form) through a heated tube, as illustrated by the following examples.

Naphthas were cracked thermally at three atmospheres total pressure, a temperature of 1500 F. and residence times of 4.3 and 4.6 seconds. The make gases were found to have characteristics tabulated as follows:

Residence Time 4.3 Sec. 4.6 See.

Gas Composition, M01 Percent:

00 0. 0 0. O 0. 0 0. 0 0. 0 0. 0 1. 8 0. 2 20. 3 18. 9 55. 2 56. 9 5. 6 5. 4 0. 6 0. 4 041110- 0. 0 0. 0 02114.- 13.5 15.4 CsHu 1. 3 1. 5 C4Ha 0. 3 0. 3 0511 0 .1 0. 0 0. 0 Diolefins and acetylene 0.5 0. 5 Aromatics 0. 9 0. 5 Heating value, B. t. u./s. 1, 064 1, 085 Specific gravity (air=1.000) 0. 61 63 Both gases were found to be absolutely interchangeable with natural gas.

Cracking of materials less volatile than straight run naphtha or crack of straight run naphtha under conditions more severe than those indicated hereinabove Will cause deposition of carbon in amounts making impracdeal the continuous cracking of the materials in a heated tube. Examples are tabulated hereinbelow of the thermal cracking of reduced crude (21.4 A. P. I., 7.04 C/H ratio and 4.4 weight per cent Conradson carbon residue) both under the conditions of the present invention and at atmospheric pressure.

Run No." 328 297 336 Light Oil Properties:

Specific Gravity, 60 F./60 F 0.862 0. 366 0.873 Index of Refraction, N13?5 C 1. 4903 1. 4918 1. 4935 Tar Properties:

Specific Gravity, 60 F./60 F 1. 096 1. 143 Viscosity at 122 F., SSU Free Carbon, Wt. Percent. Distillation, Wt. Percent Distillate- -200 C 22. 16. 2 21. 2 0300 C 41. 9 28. 9 35.0 300-355 0.. 11.4 16. 4 7.8 Pitch 47. 7 51. 9 54. 4 Sulfonation Index, Milliliters:

Unsulfonated Residue/100 Gm. Tar- For Distillate 0 300 C 1. 68 4. 00 3.14 For Distillate 300-355 0-, 0. 23 0. 65 0. 00

Run N0- 339 334 335 Operating Conditions:

Carrier as Steam Steam Steam Total Pressure, Atm. 1. 2.18 2. 13 Gauge Pressure, P. s. i. g. 17. 16. 61 Partial Pressure, Ann" O 687 1. 55 1. 57 Cracking Temp., F. l, 550 1, 540 1, 550 Residence Time, Secs 2. 2. 29 5. 65 Gals. Oil/Cu. Ft. Free Space, Hr 3.39 9. 06 3. 43 Operating Results:

B. t. u. Recovery, M B. 1:. 11./Gal 81. 79 77.01 72. 71 011 Gas Yield, 5. c. f., Ga.l 79. 06 70. 34 76. 29 Tar Yield, Wt. Percent. 32. 0 46. 7 37.1 Light 011 Yield, Wt Percent- 2.934 2.236 1.514 Carbon Yield, Wt. Percent. 11.2 s. 4 11. a Material Balance, Percent 92. 4 100.9 90. 9

Run No 339 334 335 Gas Properties:

Gas Treatment None Scrub None Scrub None Scrub Gas Composition (Diluent- Free), Vol. Percent Acetylene... 4 0.4 0.2 0.2 0.2 Butadienes. O. 6 0. 6 0. 6 O. 6 0. 2 0. 2 Pentadienes 0. l 0. 1 0. 1 0. 1 0. 1 0. 1 Benzene 1. 4 0. 6 0. 6 0. 4 0. 8 0. 3 Toluene 0.1

Total 100. 0 100. 0 100. 0 100. 0 100. 0 100. 0

Diluent-Free Heating Value,

B. 1. 11./s. c. r 1, 034 1, 017 1, 095 1,088 953 946 Spec. Gr., Diluent-Free, Air= Run No 339 334 335 Light 011 Properties:

Specific Gravity, 60 F./60 F 0.877 0.876 0.871 Index of Refraction, N D? C- 1. 4943 1. 4916 1. 4935 Tar Properties:

Specific Gravity, F./ F 1.161 1. 130 1.160 Viscosity at 122 F., SSU Free Carbon, Wt. Percent 18. 18. 95 25. 14 Distillation, Wt. Percent Distillate- 0200 C 13. 9 22. 2 15. 1 0-300 C. 34.1 35.3 26. 9 300 355 0.. 8.2 7.4 7.1 Pitch 56. 0 55. 4 62. 3 Sulfonatiou Index, Mlllllltersz Unsulfonated Residue/100 Gm. Ta.r

For Distillate 0-300 C 1. 36 1. 43 2.15 For Distillate 300355 C 0. 28 O. 29 0.00

It will be noted that when cracking was carried out at specific gravity with inert and/ or combustion gases to superatmospheric pressure, only a slightly greater amount reduce the relative burner input to 95 to 100% of the of carbon was formed, as compared with cracking at adjustment gas. atmospheric pressure to prepare an oil gas of equivalent It should be noted that in the conventional manufacture diluent-free heating value. The supcratmospheric presof oil gas having a heating value of from 1000 to 1500 sure did not affect the tar properties, except for some B. t. u./s. c. f. (at a cracking temperature of from 1350 slight shifts in the distribution of light oils and middle tar to 1700 F., at a residence time of from 1 to 5 seconds, distillates. The make gases produced at superatmospheric and at a partial pressure of the oil gasduring cracking pressures (when scrubbed to remove tars and light aroranging from 0.3 to 0.4 atmosphere), the composition of matics) were found to be absolutely interchangeable with the gas formed is fixed by the diluent (combustion blast natural gas after adjustment of the heating value and or purge gas) free heating value thereofybeing inde- Gas Composition in Percent 7 value of the diluent free oil gas is tabulated as follows:

Paraflins Due to the overlapping of the generator bricks, the brick structure forms a unified mass perforated by a large number of gas passages, with the result that the temperature therein will be substantially uniform. The

cracking of the oil and of the oil gas will, therefore, be 7 substantially uniform throughout the brick work. 7 The roofof the generator 10 is arched above the brick work and formed with a central vertical aperture 18 through which discharges an air duct 20 provided with a valve 22. An oil pipe 24 having a valve 26 and terminating in a spray device extends through the air duct 20 and the aperture 18 into the generator 10 as does also a steam pipe 30 having a valve 32.

p The roof of the superheater 12 is arched above the brick work and is formed with a central vertical vent aperture 36 closed by a valve 38. A stack is arranged above the valve 38 for removing gases discharged through the vent aperture 36. The arched roof of the superheater 12 is also formed with an aperture 42 receiving one end of a make gas exit duct 44 bent over elbow-wise to provide a dependent terminal portion dipping into sealing liquid 46 in an enclosed wash box 48. A gas duct 50 opens into the wash box 48 above the liquid level therein. The wash box 48 may be provided with a seal pot 52 and a drain pipe 54. p

The parts of the apparatus of Fig. 1 so far described are conventional. described conventional apparatus, the gas exit duct 44 is constricted, as indicated at 56,upstream of the wash box a 48 to an extent where the gas pressure generated within the generator 10 and superheater 12 (during the cracking of oil therein) will rise to superatmospheric levels while the pressure in the duct 44 downstream of the constriction 56 is not raised significantly, i. e., to an extent where the By way of a modification of the above-.

pipe 24 is opened to spray oil into the generator, and the valve 32 in the steam pipe 30 is opened. The oil is burned in a generator 10 and the combustion gases flow downwardly through the generator 10, through the duct 14, upwardly through the superheater 12 and out through the vent aperture 36 into the stack 40. If any carbon is present in the generator 10 and superheater 12 (as by way of deposit formedin a preceding make step), this carbon is then burned off. Such a heating and carbon removing step or stage is carried out in the manner conventional in connection with the operation of cyclic oil cracking apparatus of conventional construction. a

After cracking temperatures have been reached in the generator 10 and superheater 12, the valve 22 in the air duct 20 is closed. The valve 2 6 in the oil pipe 24 may then be closed for a short period of time so that the steam issuing from the pipe 30 may purge'the generator 10, the

duct 14 and the superheater 12. Next, the valve 26 in the oil pipe 24 is again opened. Then the oil sprayed into the generator 10 is cracked in the brick work therein, and the oil gas thus produced flows through the duct 14 into and through the superheater 12 where further cracking of the oil and gas takes place. But the volume of gas thus formed cannot pass through the constriction 56 in th gas duct 44 rapidly enough to prevent a substantial rise in pressure within the generator 10, the duct 14, and 'the superheater 12. Thus, cracking takes place at superatmospheric pressure, which is not effected in conventional gas making apparatus. At the same time, the Wash box 48 functions normally.

When the generator 10 and superheater 12 have cooled off in the course of the make period to a temperature at each other and two superheaters 64 and 66 disposed, re-

spectively, adjacently to the generator and a generator 62. At their bottoms, the generator 60 and the superheater 64 are interconnected by a duct 68 to form a first generator-superheater pair, while the generator 62 .and the superheater 66 are similarly interconnected by a duct 70 to form a second generator-superheater pair. The generators60 and 62 and the superheaters 64 and 66 are all provided with interior brick work 72 similar to that of the apparatus of Fig. 1. The roofs of the generators 60 and 62 are each arched,

above the brick work therein and are each formed, re-

' each formed, respectively, with central vertical apertures pressure will interfere with the normal functioning of the wash box 48. The exact size of the throat defined at constriction 56 will vary, of course, with the size and capacity of the apparatus as well as with the rate of oil gas production therein. It is within the skill of those versed in 'the art to compute the appropriate size of the orifice to Note that downstream j the vent 36, the valve 22 is opened to admit air through the du'ct 20 into the generator 10, the valve 26 in the oil spec tively, with central vertical apertures 74 and 76 receiving vertical ducts 78 and 80 opening upwardly into.

a cross duct 82. .The roofs of the superheaters 64 and 66 are likewise arched above the brick work therein and are 84 and 86 receiving vertical ducts 88 and 90 having upper openings closed by valves 92 and 94. Stacks 96 are provided above the upper ends of the ducts 88 and 90 to receive gases vented therefrom when the valves 92 and 94 are open.

An air duct 98 having a valve 100 discharges. into the duct 78 leading into the generator 60 and another air duct 102 having a valve 104 discharges into the duct 80 leading into the generator 62. An oil spray device 106 for the generator 60 extends through the duct 78 and is supplied with oil to be cracked through a pipe 108 having a valve 110 and with oil to be burned for heating through a pipe 112 having a valve 114. An oil spray device 116 for the generator 62 extends through the duct 80 and is supplied with oil to be burned for heating through a pipe 118 having a valve 120 and with oil to be cracked through a pipe 122 having a valve 124.

A gas duct 126 branches off from the duct 88 of the superheater 64 and has connected thereto an air duct 1.28. valved at 130 as well as a steam pipe 132 valved at 134. Similarly, a gas duct 136 branches off from the duct 90 of the superheater 66 and has connected thereto an air duct 138 valved at 140 as well as a steam pipe 142 valved at 144. Both ducts 126 and 136 discharge into a main gas duct 146 leading to a wash box 148 containing liquid 150 into which the lower end of the main gas duct 146 dips. Another gas duct 152 conducts the make gas from the wash box 148. A valve 154 in the top of the duct 146 serves alternately to close the orifices of the ducts 126 and 136 into the duct 146.

As so far described, the apparatus of Fig. 2 is conventional. According to the present invention, the duct 146 is constricted downstream of the valve 154 and upstream of the wash box 148, as indicated at 160. This constriction is dimensionel so as to dam up the oil gas pressure generated within the generators 60 and 62 and superheaters 64 and 66 when oil is cracked therein. Further, the conduit 146 is made suificiently wide downstream of the constriction 160 to permit expansion of oil gas passing therethrough whereby normal function of the wash box 148 is not interfered with.

In the operation of the apparatus of Fig. 2 through a four-stage regenerative cycle, a first blast period is initiated by closing the valve 92, then opening the valve 94, and moving the valve 154 to close the outlet of the duct 126. Primary air is admitted through the conduit 128 by opening the valve 130. Secondary air is admitted through the conduit 102 by opening its valve 104, and oil for heating is supplied through the pipe 118 by opening the valve 120. The primary air passes through the superheater 64 and the generator 60 wherein cracking has been eiiected in the preceding make period, and the air then removes carbon deposited in the brick work of the first generator-superheater pair while being preheated before reaching the second generator-superheater pair at the end of this blast period, the valves 104, 120, 130, and 94 are closed.

In the next succeeding make period, oil is cracked in the second generator-superheater pair. For this purpose, the valve 134 of the steam pipe 132 is opened and the valve 124 of the oil pipe 122 is opened to supply oil to the generator 62. The make gas leaves the top of the superheater 66 and passes through the ducts 136 and 146, through the wash box 148 and into the gas main 152. During this period, carbon is deposited in the brick Work of the second generator-superheater pair. At the conclusion of this make period, the valves 124 and 134 are closed and the valve 154 is moved to close the outlet of the duct 136 into the duct 146 and to open the outlet of the duct 126.

In the next succeeding blast period, the valve 92 is opened. Primary air is admitted through the pipe 138 by opening the valve 140 to remove the carbon deposits from the brick work in the generator 62 and the superheater 66, the air being preheated while passing therethrough. Secondary air is admitted from the pipe 98 by opening its valve 100 and oil is supplied from the pipe 112 by opening the valve 114. At the end of the blast period, the valves 114, 140, 100, and 92 are closed.

In the next succeeding make period, the valve 144 in the steam pipe 142 is opened and oil is supplied to the generator 60 by opening the valve 110 in the oil pipe 108. The make gas leaves the top of the superheater 64 and passes through the ducts 126 and 146, through the wash box 148 and into the gas main 152. During this period carbon is deposited in the brick work in the first generator-superheater pair. At the end of this make period valves 110 and 144 are closed and the valve 154 is moved to close the outlet of the duct 126 into the duct 146 and to open the outlet of the duct 136. The apparatus is then ready for repetition of the four-stage regenerative cycle above described.

The four-stage regenerative cycle above described is en tirely conventional except for the fact that during the two make periods, a superatmospheric pressure is maintained in the generators 60 and 62 and in the superheaters 64 and 66. As a result, the oil gas formed is nearly or completely interchangeable with natural gas.

A regenerative apparatus of somewhat difierent design is shown in Fig. 3. The apparatus there illustrated is generally U-shaped, having left and right erect shells indicated generally, respectively, at 200 and 202 and interconnected at their bottoms by a conduit 204. The shell 200 has provided therein spaced lower and upper checker brick works 206 and 208 functioning, respectively, as a generator and a superheater. The right shell 202 is provided with similarly functioning spaced lower and upper brick works 210 and 212. An oil spray device 214 pmjecting into the interspace between the generator 206 and the superheater 208 is supplied with oil to be cracked through a pipe 216 valved at 218 and with oil to be burned for heating through a pipe 220 valved at 222. An air duct 224 having a valve 226 also projects into the said interspace. Similarly, an oil spray device 228 projecting into the interspace between the generator 210 and the superheater 212 is supplied with oil to be cracked by a pipe 230 valved at 232 and with oil to be burned for heating through a pipe 234 valved at 236. An air duct 238 having a valve 240 also projects into the interspace between the generator 210 and the superheater 212.

The roof of the shell 200 is arched above the brick work 208 and is formed with a vertical aperture 242 receiving a vertical duct 244 closed upwardly by a valve 246 which when opened permits venting of the shell 200 into a stack 248. Similarly, the shell 202 is arched above the brick work 212 and is formed with a vertical aperture 250 receiving a vertical duct 252 closed upwardly by a valve 254 which when opened permits venting of the shell 202 into a stack 256. A gas duct 258 branches off from the duct 244 for the shell 200 and has an air duct 260 valved at 262 as well as a steam pipe 264 valved at 266 connected thereto. Similarly, a gas duct 270 branches ofi from the duct 252 for the shell 202 and has an air duct 272 valved at 274 as well as a steam pipe 276 valved at 278 connected thereto. Both ducts 244 and 252 discharge into a gas duct 280 having a lower end dipping into liquid 282 of a wash box 284 from which make gas is discharged into a gas main 286. A valve 288 at the top of the gas duct 286 is movable between two extreme positions closing the outlets into the gas duct 280, respectively, of the ducts 258 and 270.

As so far described, the apparatus of Fig. 3 is conventional.

According to the present invention, the gas duct 280 is constricted, as indicated at 290, downstream of the valve 288 and upstream of the wash box 284. Further, the duct 280 is sufiiciently Wide downstream of the constriction 290 to permit deceleration of the make gas issuing from the constriction 290 so that the wash box 284 will function normally. The constriction 290 serves to dam up the oil gas pressure generated within the shells 200 and 202 and the conduit 204 when cracking is carried out, with the above-noted result of making the oil gas nearly or completely interchangeable with natural gas.

In the operation of the apparatus of Fig. 3, blasting is first carried out. The valve 246 is closed, the valve 254 is opened, and the valve 288 is moved to close the outlet of the duct 258. Primary air is admitted from the duct 260 by opening the valve 262 to remove any carbon deposits formed in the superheater 208 and the generator 206 in a preceding make period, the primary air being preheated on passage through these structures. Secondary air is admitted through the duct 238 by opening its valve 240 and oil for heating is supplied by opening the valve 236 of the pipe 234. The generator 210 and superheater 212 are thus brought to cracking temper- -11 atures. At the end of the blast period the valves 236, 240, 262, and 254 are closed.

In the next succeeding make period, the valve 266 of In the next succeeding blast period, the valve 246 is opened and primary air is admitted from the duct 272 by opening the valve -274 to remove carbon deposited in the generator 210 and superheater 212, this air being preheated on passage through the brick work of these structures. Secondary air is admitted from the conduit 22 4 by opening the valve 226 and oil is supplied to the spray device 214 from the pipe 220 by opening the valve 222'; At the end'of this blast period, the valves 222, 274, 226, and 246 are closed.

' In the next succeeding make period, the valve 278 of the steam pipe 276 and the valve 218 of the oil pipe 216 are opened. The make gas leaves the top of the shell 200 and passes through the ducts'244 258, and.

280 and through the wash box 284 into the gas main 286. Carbon isdeposited in the generator 206 and superheater' 208. At the conclusion of this make period, the valves 218 and 278' are closed and the valve 288 is moved was to close the outlet of the duct 258. The apparatus'isthen ready for the repetition of theabovedescribed four-stage regenerative cycle.

Manydetails may be varied without departing from the principles of this invention, and it istherefore not my purpose to limit the patent granted .on this inven mm otherwise than necessitated by the scope of the appended claims.

, We claim: V V

1. A method of preparing an oil gas absolutely interchangeable with natural gas which comprises thermally cracking a petroleum fraction not more volatile than straight run naphtha at a total pressure of from two to four atmospheres, a residence time of from 15 to 2 seconds and a temperature'of from 1300 to 1750 F., and thereafter removing light aromatics from the resulting product gas. V

2. A cyclic, regenerative oil cracking process for mak ing an oil gas which comprises providing first, second, third and fourth gas permeable bodies capable of absorbing heat and of releasing heat on gas flow therethrough, raising said bodies to a cracking temperature of between 1300 and 1750" F., thereafter flowing a petroleum oil fraction not more volatile than straight run naphtha through said third and fourth bodies in said sequence while maintaining the resulting .vapors at' a total pressure of between two and four atmospheres and at a residence time ranging from 15 to 2 seconds, said oil being cracked to form oil gas on flow through said bodies, collecting said oil gas thus formed, thereafter flowing air'through said fourth, third, second, and first bodies in said sequence to remove carbon deposited in said third and' fourth bodies when said oil is cracked therein while flowing additional air through said second and first bodies in said sequence and flowing oil into said second body for combustion therein, said flowing of air and oil being continued until said bodies have been raised to a cracking temperature of between 1300 and 1750" F.

and the hot gases being vented after flow out of said first body, thereafter flowing said oil through said second and first bodies in said sequence'while maintaining the resulting vapors at a total pressure of between two and four atmospheres and at a residence time ranging from 15 to 2 seconds, said oil being cracked to form oil gas on flow through said bodies, collecting. the oil gas thus formed, thereafter flowing air through said first, second, third and fourth bodiesin said sequence to remove carbon deposited in said first and second bodies when said oil is cracked therein while simultaneously flowing additional air through said third and fourth bodies in. said sequence and flowing oil into said third body for combustion therein, said flow of said air and oil being continued until said bodies have been raised to a cracking temperature and hot gases being vented after. flow out of said fourth body, and thereafter repeating the cycle recited hereinabove beginning with the first recited flow ing of said oil.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Sachanen, Conversion of Petroleum, 2d edition, 1948, pages 10, 11 and 540. 

1. A METHOD OF PREPARING AN OIL GAS ABSOLUTELY INTERCHANGEABLE WITH NATURAL GAS WHICH COMPRISES THERMALLY CRACKING A PETROLEUM FRACTION NOT MORE VOLATILE THAN STRAIGHT RUN NAPHTHA AT A TOTAL PRESSURE OF FROM TWO TO FOUR ATMOSPHERES, A RESIDENCE TIME OF FROM 15 TO 2 SECONDS AND A TEMPERATURE OF FROM 1300* TO 1750* F., AND THEREAFTER REMOVING LIGHT AROMATICS FROM THE RESULTING PRODUCT GAS. 